The present invention relates to a method and to an arrangement for the avoidance of errors during the reproduction of an image pattern wherein the image pattern is arranged on a driven scanning drum which could be nonuniformly driven, is scanned by photo-electrical means point-by-point and line-by-line to generate image data to be recorded on a recording medium which, in turn, is arranged on a recording drum driven independently of the rotation of the scanning drum and possibly nonuniformly.
In one example, the method according to the present invention avoids image errors and screen distortions and finds use in a printing form engraving arrangement for gravure printing.
To begin with, the mode of operation of a printing form engraving arrangement is described and the problem to be solved is discussed.
In a scanning machine, an image pattern is mounted in a stretched-out manner on a scanning cylinder to be rotated, with the image pattern being scanned in a helicoidal manner by a light spot of scanning member which is guided in parallel to the scanning cylinder. The image pattern can contain half-tone images and lettering. Depending on the tone-value of the scanned image points, more or less light is reflected from the image pattern onto the scanning member and is photo-electrically converted to an image signal. For screening the imagine pattern, a screen timing signal is superimposed over the image signal.
In the engraving machine, an engraving member, e.g. in the form of a mechanical engraving system, moves parallel to a rotating printing cylinder and uses an engraving needle as a cutting tool. The screen timing signal and the image signal control the engraving member. The screen timing signal brings about a vibrating lifting movement of the engraving needle and during the recording process, the engraving needle cuts a series of cavities or cups, which are arranged in a ruled screen, out of the surface of the rotating printing cylinder. The image signal or the tone values of the scanned image points determine the depth of the engraved cavities.
The engraving machine can also use an electron ray or Laser beam gun. In these cases, the cavities are formed through the action of the radiation on the surface of the printing cylinder. The screen timing signal then determines the firing sequence and the image signal the intensity of the radiation.
The engraved printing cylinder is the printing form for a gravure rotary machine. Prior to the printing process, the color transmission from the cavities onto the printing carrier takes place. The tone value of a printed surface is determined by the volume of the cavities arranged in a screen-like manner in this surface and by the quantity of color stored in these cavities.
For the reproduction of a multicolored image pattern, each color extraction or color separation engraves one printing cylinder. In the gravure rotary machine the superimposed printing of the differently colored printing cylinders onto the printing medium takes place during one sequence of operation.
Especially in the multicolor printing process, a high reproduction quality requires an accurate production of the printing cylinder of a color set, with an accurate arrangement of the cavities in a ruled screen.
Some of the factors which essentially influences the accuracy of the production of the printing form, are the uniformity of the circumferential speed of a cylinder with respect of a stationary point and the synchronous and surface correspondence between the scanning cylinder and the printing cylinder.
An image defect occurs in the case of a nonuniform circumferential speed of one of the cylinders. This image defect becomes magnified when the respective circumferential speeds of scanning and printing cylinders fluctuate. An image defect manifests itself first of all in that a straight line in the image pattern undergoes a vibrating motion and is recorded on the printing cylinder in an inaccurate position.
In the case of an inaccurate positioning of the engraving onto the printing cylinder of a color set, errors in proper matching occur during the superimposed printing of several colors to form a multicolor print and considerably reduces the quality of a printing image.
The structure of a ruled screen is determined by the interrelationship between the circumferential speed of the printing cylinder, the screen timing signal and the rate of advance of the engraving member. If any one of these parameters is not stable, distortions of the ruled screen occur.
In the case in which the circumferential speed of the printing cylinder is nonuniform and the circumferential speed related to the stationary engraving member is controlled by a constant screen timing signal which, in turn, is independent of the rotational movement of the printing cylinder, the above described distortions of the ruled screen occur during the reproduction in addition to the image defects.
In order to give a concept of the required accuracies, it is pointed out here that the deviation of the location of an engraved cavity from its ideal location is not allowed to be greater than about 2 microns.
If greater periodic deviations occur and if they combine to patterns on the large surface, a moire pattern occurs during the engraving process and the human eye finds such a pattern disturbing, especially when viewing the finished printing image.
Moreover, during the joint printing of several colors to a multicolor print, the distortions of the ruled screen make themselves known in the color shift which likewise influences the quality of a print of a considerable extent. When speaking of the color shift this refers to the statistic or periodic coverage or non-coverage of printing points of different color.
Up to now, it has been assumed herein that during the reproduction of an image pattern a scanning machine works together in each case with an engraving machine.
However, when printing magazines, one is frequently faced with the problem of having to produce simultaneously several uniform printing cylinders from one image pattern or of having to produce simultaneously all four printing cylinders of a color set from one colored image pattern.
For this task, a scanning machine is combined with several engraving machines to an engraving installation.
A second scanning machine (commonly called a mask-machine) is utilized for the control of the recording and to copy lettering onto the image patterns. While the scanning drum of the first scanning machine bears the image patterns to be reproduced, a control-maks or control frame is arranged on the scanning drum of the second scanning machine. By scanning of this control mask, the necessary control signals are obtained. These working procedures likewise require an accurate synchronism of all cylinders.
In the known engraving installations, each cylinder is driven via a driving gear by a synchronous motor designed especially for very uniform rotational movements. Each synchronous motor is energized by a frequency converter. From a primary network system, a converter creates an artificial secondary network system, the frequency of which is determined by the frequency of a guidance timing signal controlling the converter. The rotational speed of the synchronous motor is proportional to the frequency of the secondary network system and therewith is also proportional to the frequency of the guidance timing signal of the converter. The advance movements of the scanning and engraving member are likewise generated by synchronous motors energized by converters. The guidance timing signals of the converter and also the screen timing sequence are derived through frequency-division from the frequency of a quartz-crystal-controlled oscillator. It is achieved thereby, that at least the rotary fields of the synchronous motors and the scanning timing sequence are in phase-locked relationship to one another. To be sure, the scanning cylinder and the printing cylinder rotate in synchronous manner, but the angle between them is not maintained as accurately, as for both cylinders coupled by a common shaft.
As is known, the rotary field and the rotor of a synchronous motor are not tightly coupled. For this reason, a synchronous motor reacts to load fluctuations with a change of its load-angle. In the case of sudden load-changes, the transition to the new operating condition takes place under oscillations of the rotor with respect to the rotary field. Load fluctuations which are based on imbalances of the driven cylinder and of the driving gear or based on nonuniform bearing-friction, in practice cannot be avoided, so that changes of the load-angle occur.
In addition, gear-wheel play in the drive unit is a source of variations. Accordingly, the angle-follow of rotational movements of scanning and engraving cylinders are not dependable and image defects occur.
No strong relationship necessary for the constructions of an accurate ruled screen exists between the scanning timing signal which determines the points of time of the engraving of the cavities, and the circumferential speed of the printing cylinders to determine the position at a given moment of the cylinder surface in relation to the engraving member.
This results in distortions of the ruled screen and leads to the mentioned defects.
It is known to use in engraving installations a special motor which responds with only a very slight change of its load-angle to load changes and is well damped, but the coupling between the rotary field and the rotor cannot be considered to be a rigid one. Furthermore, the gear wheel play is not eliminated so that despite a considerable expenditure in the driving means image defects as well as other distortions of the ruled screen can occur and thereby considerable reduce the reproduction quality. In the case of gravure, it is furthermore desired to engrave cylinders having relatively large dimensions. Higher drive powers are required to accomplish this.
Prior art systems and methods have the operating speed of an engraving installation limited by the operating speed of the mechanical engraving system, although the latter can be considerably increaded through the utilization of engraving members operating with an electron ray or a laser-beam gun for which the circumferential speeds of the cylinders is increased or the number of revolutions of the motor which activates the cylinders is increased.
In contrast, an increase of the performance and of the number of revolutions is not possible in the case of the known special motors, so that other driving means have to be utilized. Furthermore, for the drive means for the cylinders, it is desirable to strive as much as possible to utilize commercial motors which are to be energized from the electric mains, so as to reduce expenditures in time and costs incurred by special motors energized through converters.
Commercial motors, however, show fluctuations in synchronism and, when used in engraving installations, result in image defects and screen distortions for this reason.
In another known engraving installation, screen distortions are avoided by using a screen timing sequence derived from the rotational movement of the printing cylinder in connection with a timing generator, but the image defects are present therein, as in other systems. For this reason, the instant invention is based on the task of eliminating the cited disadvantages and of providing a process which, by the utilization of commercial motors for the drive of the cylinders, avoids image defects and screen distortions.